The present invention relates generally to a method of shattering earth formations by drilling boreholes in the formation and placing explosives therein which are detonated. More particularly, the invention relates to a method of plugging a borehole at any desired position in order to improve the performance of the explosives, and reduce the danger of fly rock and stemming ejection.
A conventional method of shattering earth formations in mining or excavation operations includes the steps of drilling a plurality of boreholes in the formation in a predetermined array, placing a predetermined amount of explosives in the boreholes, and detonating the explosives in a conventional sequence. Typically, the boreholes are not completely filled with explosives, but are plugged at the top by a stemming material such as crushed rock. In addition, the stemming at the tops of the boreholes may be spaced from the explosives by an air deck and/or other additional layers of air, explosives and stemming or plugging material.
Regardless of the number and arrangement of materials and decks in the boreholes, the upper layer of stemming is intended to plug the top of the borehole and confine the blast for a few extra milliseconds than would otherwise be the case in order to improve the cratering produced by the explosives and the fragmentation of material being shattered and to reduce dangerous fly rock and stemming ejection. As such, it is considered an important step in known processes to provide the stemming layer at the upper ends of the boreholes.
Although crushed rock is preferably used as the stemming material in mining and excavation operations, it is not always readily available, and must sometimes be shipped to the mining site for use. Because mining and site preparation is often conducted in urban locations, fly rock and stemming ejection can be dangerous. As such, there is a need for a stemming plug construction capable of use with readily available materials to prevent such dangerous conditions. Unfortunately, drill cuttings are much smaller than crushed rock, usually on the order of 6 mesh down to 200 mesh in size, and are easily blown from the boreholes upon detonation of the explosives. As such, very little confinement of the blast results, reducing the amount of cratering and fragmentation relative to that achieved when crushed rock is employed as the stemming material. The stemming plug in conjunction with crushed rock provides extra safety in confined areas where stemming ejection and fly rock can be dangerous.